Illustrated in U.S. Pat. No. 6,392,339 on xe2x80x9cOrganic Light Emitting Devices Having Improved Efficiency and Operation Lifetimexe2x80x9d, filed on Jul. 20, 1999 and U.S. Pat. No. 6,392,250 on xe2x80x9cOrganic Light Emitting Devices Having Improved Performancexe2x80x9d, filed on Jun. 30, 2000, the disclosures of which are totally incorporated herein by reference, are organic light emitting devices (organic EL devices) that, for example, comprise a mixed region including a mixture of a hole transport material and an electron transport material. At least one of a hole transport material region and an electron transport material region can be formed on the mixed region. The stability of the above mentioned organic EL devices disclosed in U.S. Pat. No. 6,392,339 and U.S. Pat. No. 6,392,250 is usually reduced at temperatures above 80xc2x0 C., due it is believed to a decrease in the device resistance to shorting and also since it is believed to a progressive increase in the driving voltage required to drive a certain current through the organic EL devices. As a result, the operational stability of these devices can be limited to few hundred hours or less at these high temperatures, and more specifically, at high temperatures in the range of from about 80xc2x0 C. to about 100xc2x0 C. Therefore, these devices are believed to be unsatisfactory in some instances, for applications in which there is desired an operational stability of the organic EL device of at least, for example, several thousand hours at temperatures of, for example, 90xc2x0 C., such as, for example, in some automotive, military or other industrial applications where durability in harsh conditions is desired and/or necessary.
The appropriate components and processes of the above copending applications may be selected for embodiments of the present invention in embodiments thereof.
1. Background of the Invention
This invention relates to optoelectronic devices and, more particularly, to organic light emitting devices (organic EL devices). More specifically, the present invention relates to substantially stable organic EL devices and which devices do not in embodiments, for example, usually degrade at high temperatures, such as about 100xc2x0 C., and moreover, which devices are not substantially adversely affected by high temperatures. This invention also relates in embodiments to methods for the preparation of organic light emitting devices and uses thereof.
2. Prior Art
An organic EL device can be comprised of a layer of an organic luminescent material interposed between an anode, typically comprised of a transparent conductor, such as indium tin oxide, and a cathode, typically a low work function metal such as magnesium, calcium, aluminum, or the alloys thereof with other metals. The EL device functions on the primary principle that under an electric field, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the luminescent layer and undergo recombination to form excitonic states which subsequently emit light. A number of prior art organic EL devices have been prepared from a laminate of an organic luminescent material and electrodes of opposite polarity, which devices include a single crystal material, such as single crystal anthracene, as the luminescent substance as described, for example, in U.S. Pat. No. 3,530,325. However, these devices are believed to require excitation voltages on the order of 100 volts or greater.
An organic EL device with a multilayer structure can be formed as a dual layer structure comprising one organic layer adjacent to the anode supporting hole transport, and another organic layer adjacent to the cathode supporting electron transport and acting as the organic luminescent zone of the device. Examples of these devices are disclosed in U.S. Pat. Nos. 4,356,429; 4,539,507 and 4,720,432, wherein U.S. Pat. No. 4,720,432 discloses, for example, an organic EL device comprising a dual-layer hole injecting and transporting zone, one layer being comprised of porphyrinic compounds supporting hole injection and the other layer being comprised of aromatic tertiary amine compounds supporting hole transport. Another alternate device configuration illustrated in this patent is comprised of three separate layers, a hole transport layer, a luminescent layer, and an electron transport layer, which layers are laminated in sequence and are sandwiched between an anode and a cathode. Optionally, a fluorescent dopant material can be added to the emission zone or layer whereby the recombination of charges results in the excitation of the fluorescent.
There have also been attempts to obtain electroluminescence from organic light emitting devices containing mixed layers, for example, layers in which both the hole transport material and the emitting electron transport material are mixed together in one single layer, see, for example, J. Kido et al., xe2x80x9cOrganic Electroluminescent Devices Based On Molecularly Doped Polymers,xe2x80x9d Appl. Phys. Lett. 61, pp. 761-763, 1992; S. Naka et al., xe2x80x9cOrganic Electroluminescent Devices Using a Mixed Single Layer,xe2x80x9d Jpn. J. Appl. Phys. 33, pp. L1772-L1774, 1994; W. Wen et al., Appl. Phys. Lett. 71, 1302 (1997); and C. Wu et al., xe2x80x9cEfficient Organic Electroluminescent Devices Using Single-Layer Doped Polymer Thin Films with Bipolar Carrier Transport Abilities,xe2x80x9d IEEE Transactions on Electron Devices 44, pp. 1269-1281, 1997. In a number of such structures, the electron transport material and the emitting material are the same. However, as described in the S. Naka et al. article, these single mixed layer organic light emitting devices are generally less efficient than multi-layer organic light emitting devices. Recent EL research results indicate that those devices including only a single mixed layer of a hole transport material (composed of NBP, a naphthyl-substituted benzidine derivative) and an emitting electron transport material (composed of Alq3, tris(8-hydroxyquinoline) aluminum are inherently believed to be unstable. The instability of these devices is believed to be caused by the direct contact between the electron transport material in the mixed layer and the hole injecting contact comprised of indium tin oxide (ITO), which results in the formation of the unstable cationic electronic transport material, and the instability of the mixed layer/cathode interface, see H. Aziz et al., Science 283, 1900 (1999), the disclosure of which is totally incorporated herein by reference.
Also, there have been attempts to obtain electroluminescence from organic light emitting devices by introducing a hole transport material and an emitting electron transport material as dopants in an inert host material, as reported in the above-described article by J. Kido et al. However, such known devices have been found to be generally less efficient than conventional devices including separate layers of hole transport material and emitting electron transport material.
While recent progress in organic EL research has perhaps elevated the potential of organic EL devices, the operational stability of current available devices may still be below expectations. A number of known organic light emitting devices have relatively short operational lifetimes before their luminance drops to some percentage of its initial value. Although known methods of providing interface layers as described, for example, in S. A. Van Slyke et al., xe2x80x9cOrganic Electroluminescent Devices with Improved Stability,xe2x80x9d Appl. Phys. Lett. 69, pp. 2160-2162, 1996, and doping as described, for example, Y. Hamada et al., xe2x80x9cInfluence of the Emission Site on the Running Durability of Organic Electroluminescent Devicesxe2x80x9d, Jpn. J. Appl. Phys. 34, pp. L824-L826, 1995 may perhaps increase the operational lifetime of organic light emitting devices for room temperature operation, the effectiveness of these organic light emitting devices deteriorates dramatically for high temperature device operation. In general, device lifetime is reduced by a factor of about two for each 10xc2x0 C. increment in the operational temperature. Moreover, at these high temperatures, the susceptibility of the organic light emitting devices is increased, as described, for example, in Zhou et al., xe2x80x9cReal-time observation of temperature rise and thermal breakdown processes in organic LEDs using an IR imaging and analysis systemxe2x80x9d, Advanced Materials 12, pp 265-269, 2000, which further reduces the stability of the devices. As a result, the operational lifetime of known organic light emitting devices at a normal display luminance level of about 100 cd/m2 is limited, for example, to about a hundred hours or less at temperatures in the range of 60xc2x0 C. to 80xc2x0 C., reference J. R. Sheats et al., xe2x80x9cOrganic Electroluminescent Devices,xe2x80x9d Science 273, pp. 884-888, 1996, and also S. Tokito et al., xe2x80x9cHigh-Temperature Operation of an Electroluminescent Device Fabricated Using a Novel Triphenlamine Derivative,xe2x80x9d Appl. Phys. Lett. 69,878 (1996).
This invention overcomes or minimizes many of the above-described disadvantages with regard to a number of existing organic light emitting devices (OLEDs) and provides in embodiments organic light emitting devices with enhanced operational lifetimes. In addition, the organic light emitting devices according to embodiments of the present invention can provide operational stability at high temperatures, such as, for example, an operational lifetime of several hundreds of hours, such as 1,200 hours up to several thousands of hours, such as about 10,000 hours for typical display luminance of about 100 cd/m2 at temperatures of from about 80xc2x0 C. to about 100xc2x0 C. and above, device operation conditions. Accordingly, the organic light emitting devices of the present invention can be used for various numerous applications, and especially high temperature technological applications that usually require high temperature stability over long periods of times, such as, for example, about 500 to about 12,000 hours.
Organic light emitting devices according to this invention comprise, for example, in sequence;
(i) a substrate;
(ii) a first electrode;
(iii) a mixed region comprising a mixture of a hole transport material and an electron transport material, and wherein this mixed region comprises an organic luminescent material;
(iv) a second electrode;
(v) a thermal protective element coated on the second electrode wherein one of the two electrodes is a hole injection anode, and one of the two electrodes is an electron injection cathode, and wherein the organic light emitting device further comprises;
(vi) a hole transport region, interposed between the anode and the mixed region, wherein the hole transport region includes a buffer layer; and
(vii) an electron transport region interposed between the cathode and the mixed region.
The mixed region, the hole transport region including the buffer layer, and the electron transport region can minimize or reduce changes in device luminance and/or driving voltage during device operation, and enable stability in the device luminance and/or driving voltage during device operation for extended periods of time at elevated temperatures, while the thermal protective coating increases the device resistance to shorting at elevated temperatures, and thus improves, for example, the thermal durability of the organic EL device.
In embodiments, the organic light emitting devices can further include an electron injection layer interposed between the electron transport region and the cathode. The electron injection layer can function to improve the injection of electrons from the cathode into the electron transport region, and therefore, the efficiency of the organic light emitting devices is increased.
An organic light emitting device according to this invention comprises a substrate, an anode laminated on the substrate, a hole transport region laminated on the anode, a buffer layer preferably situated in the hole transport region and in contact with the anode, a mixed region comprising a mixture of a hole transport material and an electron transport material laminated on the hole transport region, wherein the mixed region comprises an organic luminescent material, an electron transport region laminated on the mixed region, a cathode laminated on the electron transport region, and a thermal protective element or layer of, for example, a silicon oxide, inclusive of silicon oxides and silicon dioxides laminated on the cathode. Moreover, in embodiments the organic light emitting device according to this invention comprises a substrate, a cathode laminated on or situated upon the substrate, an electron transport region laminated on the cathode, a mixed region comprising a mixture of a hole transport material and an electron transport material and wherein the mixed region comprises an organic luminescent material, a hole transport region laminated on the mixed region wherein this region includes a buffer layer, an anode laminated on the hole transport region; and a thermal protective element laminated on the anode, wherein at least one of the hole transport material and the electron transport material comprising the mixed region is an organic luminescent material, wherein the mixed region further includes an organic luminescent material as a dopant, and wherein at least one of the hole transport material and the electron transport material in the mixed region can also be a luminescent material. The organic light emitting device can further comprise an electron injection layer interposed between the electron transport region and the cathode, and methods thereof, such as forming a mixed region comprising a mixture of a hole transport material and an electron transport material, wherein the mixed region contains an organic luminescent material, and wherein the mixed region is situated between a first electrode and a second electrode, and wherein the second electrode is coated with a thermal protective element, a hole transport region which includes a buffer layer and an electron transport region can be formed on the mixed region; and an optional electron injection layer in contact with the electron transport region, one of the first and second electrodes can be formed in contact with the hole transport region and serves as an anode and one of the first and second electrodes is formed in contact with either the electron transport region or the electron injection layer and serves as a cathode.